Reference

The networking glossary

Every term the site uses, in plain English, each linked to the chapter that explains it properly. Type in the box to filter; it searches the definitions too, so "relay" finds DERP and TURN. Below the list: the address ranges and port numbers worth knowing by heart.

4,294,967,296possible IPv4 addresses (2³²), which is why NAT exists
17,891,328private IPv4 addresses in the three RFC 1918 ranges combined
65,535usable port numbers per protocol, per address (port 0 is reserved)

Common mix-up: a MAC address and an IP address do different jobs. The MAC address gets a frame across one local link and is replaced at every router. The IP address names the final destination and stays the same end to end (unless NAT rewrites it). Your laptop's IP changes when you move to a new network; its MAC normally doesn't.

Glossary

Terms, A to Z

Numbers worth knowing

Special address ranges

If an address falls in one of these blocks, it tells you something before you look anything up.

BlockWhat it isDefined in
10.0.0.0/8Private: 16,777,216 addresses, common in offices and VPNsRFC 1918
172.16.0.0/12Private: 172.16.0.0 to 172.31.255.255; Docker and Hyper-V like itRFC 1918
192.168.0.0/16Private: the home-router favoriteRFC 1918
100.64.0.0/10Shared address space for carrier-grade NAT; Tailscale also uses it for tailnet addressesRFC 6598
169.254.0.0/16IPv4 link-local: self-assigned when DHCP failsRFC 3927
127.0.0.0/8Loopback: the machine itself (127.0.0.1)RFC 1122
192.0.2.0/24
198.51.100.0/24
203.0.113.0/24		Documentation only: never routed, safe in examples. Every "public" address on this site comes from here.RFC 5737
fe80::/10IPv6 link-local: every IPv6 interface has oneRFC 4291
fc00::/7IPv6 unique local addresses, the rough equivalent of private rangesRFC 4193
2001:db8::/32IPv6 documentation prefixRFC 3849
Numbers worth knowing

Ports you'll meet

PortProtocolUsed by
22TCPSSH, remote shell and file copy
53UDP, TCPDNS
67 / 68UDPDHCP server / client
80 / 443TCP (443 also UDP for HTTP/3)HTTP / HTTPS
123UDPNTP, time sync
179TCPBGP sessions between routers
1900UDPSSDP, the discovery half of UPnP
3389TCP, UDPRemote Desktop (RDP)
3478UDP, TCPSTUN and TURN
5351UDPNAT-PMP and PCP, sent to the gateway
41641UDPTailscale's default WireGuard port
49152–65535TCP, UDPIANA's dynamic (ephemeral) range
Sources

Where the definitions come from

  1. IETF RFCs via the RFC Editor: 768 (UDP), 791 (IPv4), 792 (ICMP), 826 (ARP), 1034/1035 (DNS), 1191 (Path MTU), 1918 (private addresses), 2131 (DHCP), 2328 (OSPFv2), 3022 (NAT), 3927 (link-local), 4193 (ULA), 4271 (BGP-4), 4291 (IPv6 addressing), 4632 (CIDR), 4787 (NAT behavior), 5389 (STUN), 5737 and 3849 (documentation addresses), 6598 (shared address space), 6886 (NAT-PMP), 6887 (PCP), 8200 (IPv6), 8445 (ICE), 8656 (TURN), 9293 (TCP).
  2. IEEE 802 standards: 802.3 (Ethernet), 802.11 (Wi-Fi), 802.1Q (VLANs and bridging).
  3. IANA: IPv4 Special-Purpose Address Registry; Service Name and Transport Protocol Port Number Registry.
  4. WireGuard protocol; Tailscale: DERP servers, How NAT traversal works.
  5. Microsoft Learn: Hyper-V virtual switches.